Considerable effort has been directed at enhancing the cutting efficiency of band saws, particularly when cutting hard metallic workpieces in production cutting applications. As the material becomes relatively hard and the number of pieces to be cut increases, the importance of operating the band saw in a manner which is highly efficient increases dramatically.
Modern production band saw controls have become quite sophisticated. In U.S. Pat. No. 4,901,612 to Harris, for example, control of the blade cutting force on the workpiece independently of the feed rate is disclosed. This control system allows the user to set a maximum cutting blade force that can be achieved below a force which will endanger the blade and then to independently set the feed rate.
When a band saw of the type disclosed in Harris U.S. Pat. No. 4,901,612 is used to cut a workpiece or bar stock having a non-rectangular cross-section, setting the blade cutting force at a constant value will automatically cause the blade feed rate to vary so as to accommodate changes in the length of the bar stock cross section. It is well known, therefore, to use a constant force controller as a means of accommodating variable workpiece cross section so as to achieve efficient cutting.
There are phenomenon, however, which will cause a constant force controlling of a band saw to produce changes in the feed rate which are not proportional to changes in the length of the cross section being cut. Thus, blade dulling will cause the workpiece to appear to have a longer cross section which requires slowing down of the feed rate. In fact, however, as the blade dulls, the feed rate needs to be increased in order to maintain the same cutting rate for the material.
Similarly, as the material becomes harder, low feed rates tend to produce work hardening of the material being cut, rather than cutting. Again, the constant cutting force controlled saw will slow down as the material work hardens and gives the appearance of cutting a longer cross sectional length. Instead, the feed rate needs to be increased, or the force increased, if work hardening occurs.
One approach to overcoming the inefficiencies which can result from merely using a constant force to control cutting is to input information as to the cross-sectional shape of the workpiece and through computer control force the saw to cut at a constant cutting rate for the shape of the workpiece. In U.S. Pat. No. 4,437,367 to Hauser, for example, a computer/controller is input with a selected one of a plurality of typical cross sectional shapes for bar stock. The controller also is input with the type of bar stock material. The controller then calculates a theoretical feed force as a function of blade position and thus workpiece cross sectional length. This theoretical feed force is then compared against an actual, sensed feed force, and the computer controls the feed force during operation to match the theoretical value for the cross section involved.
Similarly in U.S. Pat. No. 4,432,260 to Sarurai, et al., sensors are used to measure the height and width of the piece of bar stock. The computer/controller for the band saw is then input with data as to the material being cut and the shape or cross section of the bar stock. Stored in the memory of the computer/controller are cutting rates for various materials. Accordingly, the computer/controller calculates a theoretical feed rate for the various cross sectional positions as the blade cuts through the bar stocks so as to maintain the material cutting rate at about the theoretical desired value. Controlling in this fashion allows the controller to maintain higher feed rates even though the blade may be dulling or the workpiece may be experiencing some work hardening.
The problem presented by approaches such as that of the Hauser and Sarurai, et al. patents is that bar stock does not always come in simple geometric cross sections. Even shapes which are relatively simple, such as I-beams and H-beams, have sloping flanges and webs which make input to the controller as to shape only an approximation. For complex cross sections, the shape input usually has to be a rectangular or other simplified approximation. Moreover, while highly skilled operators can make adjustments and good approximations, production sawing is often being undertaken now by personnel who are not journeymen machinists or metal cutters.
Accordingly, it is an object of the present invention to provide a band saw apparatus and cutting method which is particularly well suited for highly efficient production cutting of metal bar stock by relatively unskilled operators.
Another object of the present invention is to provide a band saw and production cutting method which is capable of cutting material at a substantially constant material cutting rate for bar stock of virtually any cross section shape.
Still another object of the present invention is to provide a method of cutting workpieces from bar stock of substantially uniform cross section which does not require operator input as to the bar stock shape or cutting cross sectional length.
Another object of the present invention is to provide a band saw suitable for production cutting of a plurality of pieces from bar stock of substantially uniform cross section with minimal operator input.
Still a further object of the present invention is to provide a method and apparatus for production cutting of bar stock with a band saw which is more economical, requires minimum training and supervision of operators, and can be retro-fit to control most existing band saws.
The band saw cutting method and band saw apparatus of the present invention have other objects and features of advantage which will become apparent from, and are set forth in more detail in, the accompanying drawing and following description of the best mode of carrying out the invention.